1. Field of the Invention
The present invention concerns an arrangement to detune a reception antenna in a local coil of the type employing a switchable tuning circuit.
2. Description of the Prior Art
In modern magnetic resonance systems, magnetic resonance examination signals are acquired with the use of local coils placed close to the patient.
In addition to actual reception antennas (that are preferably fashioned as loop antennas), the local coil contains additional low-noise preamplifiers in order to be able to amplify the acquired magnetic resonance signals. The local coil additionally contains for each loop antenna, an associated active detuning circuit with which that reception antenna can be detuned during a transmission phase.
During the transmission phase the detuning is necessary in order to prevent an unwanted inductive injection of strong currents (which could be generated in the reception antenna by a received transmission signal) into the patient body. The unwanted reception of the transmission signal in the reception antenna is thus prevented with the use of the detuning circuit.
FIG. 5 shows an arrangement to detune a reception antenna LA of a local coil that is designed as a loop antenna.
The reception antenna LA shows four capacitances C1 through C4 that are fashioned to shorten the reception antenna LA.
A first capacitor C1 together with a first inductance L51 forms a block circuit that, together with a first diode D51, a second diode D52 and a second inductance L52 forms a detuning circuit VSS.
The first diode D51, the second diode D52 and the second inductor L52 are thereby connected in parallel with one another, and the two diodes D51, D52 are connected with opposite polarity relative to one another.
This parallel circuit is connected at the input side with a first end of the first capacitor C1 while the parallel circuit is connected at the output side via the first inductance L51 with a second end of the first capacitor C1.
As described, during the transmission phase it is necessary to detune the reception antenna LA. For this purpose a controllable switch DCD is provided that is connected parallel to the two diodes D51, D52.
The switch DCD is preferably designed as a DC diode and is switched with the aid of a direct current signal DCSS as a control signal. The direct current signal DCSS as a control signal is directed via cables to the switch DCD.
Given a closed switch DCD, the first inductance L51 together with the first capacitance C1 forms a high-resistance block circuit, such that the detuning circuit VSS “detunes” and the loop-shaped reception antenna LA is therefore quasi-interrupted. In this case no reception of magnetic resonance signals is possible via the reception antenna LA.
If the first capacitance C1 has a residual charge, or that the reception antenna LA nevertheless receives during the transmission phase (for example because the control signal DCSS is not present due to a line disconnection), either the first diode D51 or the second diode D52 (depending on polarity of the capacitance C1) is switched through, such that the first inductance L51 together with the first capacitance C1 forms a high-resistance block circuit. The detuning circuit VSS is therefore also “detuned” for these cases.
The second inductance L52 serves to discharge residual voltages of the two diodes D51 and D52. This type of detuning is designated as “active detuning”.
Given such local coils the respective preamplifier and the respective detuning circuits are supplied via cables with operating voltage and operating power.
The received magnetic resonance signals are also forwarded on via cables for additional processing.
The DC control signals described above for the active detuning circuit are transferred via cables.
Due to this multiplicity of necessary cables, a cable bundle connected with the local coil is formed that is unwieldy and can be difficult to manage.
Due to the dimensions and the resulting weight, as well as the movement of the cable bundle, mechanical defects can occur at the respective plugs and at the individual cables.
Concepts are presently being developed that enable a wireless transmission of the received magnetic resonance signals from the local coil for additional processing.
The received magnetic resonance signals are thereby amplified only after a successful wireless transmission, such that the preamplifiers are no longer an integrated part of the local coil.
The mechanically susceptible cable connections that are required for the feed of the control signals for the active detuning circuit still remain.